Alas, what you can and can't figure isn't dispositive. They're both fine, either "He's in the same school as John [is in]" or "He's in the same school as John's [school]."
– deadratNov 25 '15 at 8:50

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@deadrat: can you give an example of a normal-sounding sentence that uses the second construction? ?"He's in the same school as mine" sounds equally bad to me. Does it really sound fine to you?
– sumelicNov 25 '15 at 8:52

@sumelic "Normal sounding"? What's the definition of that? But, I'll try. How about "You hold an opinion that's not the same as mine"?
– deadratNov 25 '15 at 8:56

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That's slightly different, though. There is no noun after the word "same." That sentence does sound acceptable to me, but something like ?"Do you hold the same opinion as mine?" is not acceptable in my judgement.
– sumelicNov 25 '15 at 8:57

@deadrat: However, it does seem that "the same opinion as mine" is a phrase that's used in real life, and that is parallel in structure to "the same school as John's."
– sumelicNov 25 '15 at 9:00